1. Field of the Invention
The present invention generally relates to tools for use in oil and gas well operations, and more particularly to improvements to bumper spring tools for use in oil and gas wells.
2. Background of the Invention and Description of the Prior Art
A newly drilled and completed well typically has enough pressure within the formation to cause liquids in the formation and the well to flow to the surface without aid. Over time, however, as the well's production volume and bottom-hole pressure decline the liquids fall back on the perforations—the passages into the formation—thus creating what is called a “loaded well” condition. In this condition the well no longer has sufficient pressure to cause the liquids to flow to the surface without some artificial lift.
A plunger lift is a type of artificial lifting device utilized in oil and gas wells to efficiently unload liquids. The system usually requires no external energy to provide the necessary pressure to lift the liquids to the surface, instead relying on the residual pressure in the well to lift the plunger.
The gas-to-liquid ratiorequired varies depending on many conditions. The common rule of thumb used in the industry is 300 to 400 scf per barrel per 1000′ of depth.
A conventional plunger lift system for controlling production typically comprises the following structures. A well is formed by a casing that lines the well. Within the casing is a tubing string that lines the well bore through which oil or gas is produced from a formation through perforations. Within the well bore is a bumper spring assembly resting in a seating nipple or, alternatively, in a tubing or collar stop or hold down device as used in lieu of a seating nipple. A lift or bypass plunger, shown traveling upward under the pressure of the fluids and/or gas in the well bore, pushes or lifts a “slug” of fluid ahead of it. The weight of the fluid that lift or bypass plunger may lift depends on a variety of factors. Thus, it is important that other devices used in conjunction with bumper springs and lift plungers be appropriately matched to utilize the capabilities of all components in the downhole “system” so that they operate together for maximum efficiency and reliability. A typical well also includes the wellhead apparatus disposed on the surface of the earth for directing the production of the well to appropriate receptacles or pipelines.
A bumper spring assembly is a tool that is typically placed in a seating nipple at the lower end of the tubing in the well to absorb the momentum of the bypass or lift plunger as it reaches the seating nipple, thereby protecting the seating nipple from damage. Structurally, most bumper springs comprise a shaft or mandrel, a head piece at the upper end and a cage attached to the lower end. The head piece and cage are typically threaded onto the mandrel and secured with a pin to prevent the rotation of the end piece with respect to the mandrel so that the bumper spring does not become disassembled. Other methods to prevent loosening of the end pieces include welding and lock nuts.
In a typical well fluids may accumulate above the bumper spring assembly and impede production. The use of a ball-and-seat valve at the bottom of the well that opens during production flow but lacks the capability to release fluids above it when production ceases is one such example. Another example is to provide bypass relief around the ball-and-seat portion of the valve by notching the valve seat so that some of the fluid will always seep back into the formation during a close cycle. The only control of such method is the size of the notches in the valve seat, which is a poor compromise of valve effectiveness at best because it allows no control over the rate of flow during the release phase.
Some control may be provided by adding a coil spring below the ball-and-seat valve that compresses somewhat in response to fluid accumulation above the valve. When the accumulated fluid exceeds the tension in the spring, the valve opens to release fluid into the formation. A disadvantage of this type of assembly is that lack of a sleeve to stabilize the spring exposes the spring, which may typically be a wave spring—a coil spring wound of flat spring stock, to strong forces during production that may damage the spring. This lack of protection for the spring subjects the spring to reduced effectiveness in controlling release of accumulated fluids. Further, if the spring fails or collapses due to damage, there is no control of fluid release when production flow ceases. Other release valve assemblies include a sleeve inside the coil spring to stabilize it when the valve is open, and an O-ring around a floating valve seat to stabilize and seal the motion of the seat along the sleeve. While some protection and efficiency is gained, the release control is limited and the spring may still be subjected to severe forces. Moreover, the spring remains subject to failure from debris accumulation (e.g., sand and other solid matter).
Accordingly there is a need for an improved automatic release valve mechanism that provides both efficient automatic control of the release function and structural durability during use in the severe downhole environment.